Single ended amplier

[Mark Hennessy]

Hi.
In the recent overhaul and rebuild of the World designs amplifier It didn't have any form of volume control it relied on the 1 meg grid resistor and 4.7k grid stopper.
During the time I did the work I didn't have the customers preamp.
When he got it back I got a call from him he told me the Bass was brilliant. A few weeks later he came back with the amp, speakers and preamp for an intermittency on the left hand channel (turned out to be his £200 phono leads) but I had a look in the preamp (solid state) and I was surprised to see the output coupling caps were measly 0.047uf. This of course was one of the reasons when the amp had been butchered on the few occasions it worked produced poor Bass.............. a 47k across the inputs. Now don't ask me the make of the preamp all I can say it was well over a £1000 and was designed for use with valve monoblocks.

I accept that a high value volume pot "can" lead to hum pick up especially in designs that may have a far from ideal layout but my point is that in high impedance inputs a 50k would be the wrong choice.

As I said earlier:

...LF roll-off in this context is academic with modern gear. Different if you're building an amp to work with valve sources (that are often too mean to include a cathode follower and a decently sized output cap)...

So whatever this preamp was, the designer was clearly too mean to include a decently sized output cap

That is simply bad design - it's very wrong to make assumptions about the following equipment, especially in something that was clearly designed this side of 1980. It happens, sure, but it's bad. I wonder what Zout of the preamp was? Hopefully not so high as to cause HF loss in the interconnect...

Incidentally, the Quad FM4 is well-known for its miserly output cap: http://www.audiomisc.co.uk/QuadFM4/FM4.html

Yesterday I quickly knocked up an amp using the values the OP has used and yes it does work but I found that using line level there is no headroom at all and there was some instability, Raising the 12k to just 47k and increasing the cathode a couple of hundred ohms cured the instability and improved the drive no end, it still is bang on in the middle of the curve and on my distortion meter the overall output at 3 watts was under 10%, later on I reverted to an ECC83 and higher anode load with some NFB and the distortion was less than 5% the values were selected for best linearity.
The HT was 280v in this lash up.

This is really interesting - thanks for doing this

By "no headroom at all", do you mean the triode section was clipping before (or at the same time as) the EL84?

What do you mean by instability? Oscillation? What sort of frequency?

The distortion is a bit better than I'd expected.

I think Andy's done a really good job here, and it's obvious he's thought everything through in detail. And now that he's starting to tame this beast, we can discretely mention that the downside with his carefully selected triode stage is there is no scope to use negative feedback to linearise it. The distortion readings from your work certainly show a worthwhile improvement from the use of NFB.

From those that know, any comments about the original transformer? E.g., is it actually intended for push-pull operation?


Lawrence and Kalee20,

Thanks for your posts, which are helping to make things clearer for me. I've always struggled to interpret load-line graphs, partly because I've never see the value of them for transistors (to the extent that I've often wondered if they are a "hangover" from the valve days, where the load-lines are clearly very different, and there is information to be found there).

What I'd really like to see is a graph of gm verses Ia. Do these exist? Perhaps I'm too set in my "transistor ways", where gm is basically 35 times the collector current (simplification, but good enough for all practical purposes IME). And I'm spoilt by bipolar transistors, which have so much more gm compared to anything else

I've made a bit of a boo boo when I said the OP of the ECC82 is 25v, it is 25v but that's P-P, so that would be 8.8v rms.

Worry not - there's no boo boo, Andy. Your diagram clearly says 25V pk-pk

Keep at it!